Formulation of problem In recent years it has become evident that some types of antidepressants have a deleterious effect on bone metabolism with serious clinical consequences such as increased risk of fractures possibly with subsequent complications. Moreover, the majority of patients treated with antidepressants have residual symptoms including tiredness, muscle weakness and muscle pain, which are similar to symptoms of vitamin D deficiency. Increasing awareness and knowledge of the side effect profile of antidepressants on bone metabolism may prompt clinicians to screen patients at high risk of drug-induced osteopenia or osteoporosis and accordingly provide treatment, which may reduce the incidence of potentially avoidable fractures. Moreover, some types of antidepressants may show to produce a minimal or even no effect on bone turnover, and should be considered as first line treatment in the group of patients at risk of fractures. Therefore, this study, which consists of a cross-sectional and longitudinal study design, will address the following aims through biochemical and clinical characterisation: 1) to unravel underlying mechanisms leading to skeletal impairment and musculoskeletal symptoms or alterations in sensory-motor interactions (such as impaired balance) in patients receiving different types of antidepressants (selective serotonin reuptake inhibitors (SSRIs) or other newer antidepressants and 2) to investigate the effect of co-administrating vitamin D on bone status, sensory-motor interaction, and parameters associated with quality of life and depression in a randomised controlled clinical trial.